1. Field of the Invention
The present invention relates to a projection exposure method and a projection exposure apparatus. In particular, the present invention relates to a projection exposure method and a projection exposure apparatus for exposing a photosensitive substrate by projecting an image of a pattern formed on a mask onto the photosensitive substrate by the aid of a projection optical system. Especially, the present invention relates to a projection exposure method and a projection exposure apparatus for exposing a photosensitive substrate by overlaying and transferring images of patterns formed on a plurality of masks onto a predetermined area on the photosensitive substrate.
2. Description of the Related Art
Various exposure apparatuses have been hitherto used, for example, when semiconductor elements or liquid crystal display elements are produced by means of the photolithography step. At present, a projection exposure apparatus is generally used, in which an image of a pattern formed on a photomask or reticle (hereinafter generally referred to as "reticle") is transferred via a projection optical system onto a substrate (hereinafter referred to as "photosensitive substrate", if necessary) such as a wafer or a glass blade applied with a photosensitive material such as photoresist on its surface. In recent years, a reduction projection exposure apparatus (so-called stepper) based on the so-called step-and-repeat system is predominantly used as the projection exposure apparatus, in which a photosensitive substrate is placed on a substrate stage which is movable two-dimensionally, and the photosensitive substrate is moved in a stepwise manner (subjected to stepping) by using the substrate stage to repeat the operation for successively exposing respective shot areas on the photosensitive substrate with the image of the pattern formed on the reticle.
Recently, a projection exposure apparatus based on the step-and-scan system (scanning type exposure apparatus as described, for example, in Japanese Laid-Open Patent Publication No. 7-176468 corresponding to U.S. Pat. No. 5,646,413), which is obtained by applying modification to the stationary type exposure apparatus such as the stepper, is also used relatively frequently. The projection exposure apparatus based on the step-and-scan system has, for example, the following merits. That is, (1) the projection optical system is easily produced because a large field can be exposed by using a smaller optical system as compared with the stepper, and a high throughput can be expected owing to the decrease in number of shots because a large field is exposed. Further, (2) an averaging effect is obtained owing to relative scanning for the reticle and the wafer with respect to the projection optical system, and it is possible to expect improvement in distortion and depth of focus. Moreover, it is considered that the scanning type projection exposure apparatus will be predominantly used in place of the stepper, because a large field will become essential in accordance with the increase in the degree of integration of the semiconductor element, which is 16 M (mega) at present and will become 64 M for DRAM, 256 M, and 1 G (giga) in future as the progress proceeds along with times.
When the photosensitive substrate is subjected to exposure by using the projection exposure apparatus as described above, it has been contemplated to improve the resolution and the depth of focus for a pattern to be formed, by using the modified illumination method, for example, the SHRINC (Super High Resolution by Illumination Control) method as described in Japanese Laid-Open Patent Publication No. 4-273245.
Such a projection exposure apparatus is principally used as a mass-production machine for semiconductor elements or the like. Therefore, the projection exposure apparatus necessarily required to have a processing ability that how many sheets of wafers can be subjected to the exposure process for a certain period of time. That is, it is necessarily required for the projection exposure apparatus to improve the throughput.
In this context, in the case of the projection exposure apparatus based on the step-and-scan system, when a large filed is exposed, the improvement in throughput is expected because the number of shots to be exposed on the wafer is decreased as described above. However, since the exposure is performed during movement at a constant velocity in accordance with synchronized scanning for the reticle and the wafer, it is necessary to provide acceleration and deceleration areas before and after the constant velocity movement area. If a shot having a size equivalent to a shot size of the stepper is exposed, there is a possibility that the throughput is rather decreased as compared with the stepper.
The outline of the flow of the process in such a projection exposure apparatus is as follows.
(1) At first, a wafer load step is performed, in which a wafer is loaded on a wafer table by using a wafer loader.
(2) Next, a search alignment step is performed, in which the position of the wafer is roughly detected by using a search alignment mechanism. Specifically, the search alignment step is performed, for example, on the basis of the contour of the wafer, or by detecting a search alignment mark on the wafer.
(3) Next, a fine alignment step is performed, in which the position of each of the shot areas on the wafer is accurately determined. In general, the EGA (enhanced global alignment) system is used for the fine alignment step. In this system, a plurality of sample shots included in the wafer are selected beforehand, and positions of alignment marks (wafer marks) affixed to the sample shots are successively measured. Statistical calculation based on, for example, the so-called least square method is performed on the basis of results of the measurement and designed values of the shot array to determine all shot array data on the wafer (see, for example, Japanese Laid-Open Patent Publication No. 61-44429 corresponding to U.S. Pat. No. 4,780,617). In this system, it is possible to relatively accurately determine the coordinate positions of the respective shot areas at a high throughput.
(4) Next, an exposure step is performed, in which the image of the pattern on the reticle is transferred onto the wafer via the projection optical system while successively positioning the respective shot areas on the wafer to be located at exposure positions on the basis of the coordinate positions of the respective shot areas having been determined in accordance with the EGA system or the like described above and the previously measured baseline amount.
(5) Next, a wafer unload step is performed, in which the wafer on the wafer table having been subjected to the exposure process is wafer-unloaded by using a wafer unloader. The wafer unload step is performed simultaneously with the wafer load step (1) described above in which the exposure process is performed. That is, a wafer exchange step is constructed by the steps (1) and (5).
As described above, in the conventional projection exposure apparatus, the roughly classified four operations are repeatedly performed by using one wafer stage, i.e., wafer exchange.fwdarw.search alignment.fwdarw.fine alignment.fwdarw.exposure.fwdarw.wafer exchange.
The throughput THOR [sheets/hour] of such a projection exposure apparatus can be represented by the following expression (1) assuming that the wafer exchange time is T1, the search alignment time is T2, the fine alignment time is T3, and the exposure time is T4. EQU THOR=3600/(T1+T2+T3+T4) (1)
The operations of T1 to T4 are executed repeatedly and successively (sequentially) as in T1.fwdarw.T2.fwdarw.T3.fwdarw.T4.fwdarw.T1 . . . . Accordingly, if the individual elements ranging from T1 to T4 involve high speeds, then the denominator is decreased, and the throughput THOR can be improved. However, as for T1 (wafer exchange time) and T2 (search alignment time), the effect of improvement is relatively small, because only one operation is performed for one sheet of wafer respectively. As for T3 (fine alignment time), the throughput can be improved if the sampling number of shots is decreased in the case of the use of the EGA system, or if the measurement time for a single shot is shortened. However, on the contrary, the alignment accuracy is deteriorated. Therefore, it is impossible to easily shorten T3.
On the other hand, T4 (exposure time) includes the wafer exposure time and the stepping time for movement between the shots. For example, in the case of the scanning type projection exposure apparatus based on, for example, the step-and-scan system, it is necessary to increase the relative scanning velocity between the reticle and the wafer in an amount corresponding to the reduction of the wafer exposure time. However, it is impossible to easily increase the scanning velocity because the synchronization accuracy is deteriorated.
Important conditions for such a projection exposure apparatus other than those concerning the throughput described above include (1) the resolution, (2) the depth of focus (DOF: Depth of Focus), and (3) the line width control accuracy. Assuming that the exposure wavelength is .lambda., and the numerical aperture of the projection lens is N.A. (Numerical Aperture), the resolution R is proportional to .lambda./N.A., and the depth of focus (DOF) is proportional to .lambda./(N.A.).sup.2.
Therefore, in order to improve the resolution R (in order to decrease the value of R), it is necessary to decrease the exposure wavelength .lambda., or it is necessary to increase the numerical aperture N.A. Especially, in recent years, semiconductor elements or the like have developed to have high densities, and the device rule is not more than 0.2 .mu.m L/S (line and space). For this reason, a KrF excimer laser is used as an illumination light source in order to perform exposure for the pattern. However, as described above, the degree of integration of the semiconductor element will be necessarily increased in future. Accordingly, it is demanded to develop an apparatus provided with a light source having a wavelength shorter than that of KrF. Representative candidates for the next generation apparatus provided with the light source having the shorter wavelength as described above include, for example, an apparatus having a light source of ArF excimer laser, and an electron beam exposure apparatus. However, the case of the ArF excimer laser involves numerous technical problems in that the light is scarcely transmitted through a place where oxygen exists, it is difficult to provide a high output, the service life of the laser is short, and the cost of the apparatus is expensive. The electron beam exposure apparatus is inconvenient in that the throughput is extremely lowered as compared with the light beam exposure apparatus. In reality, the development of the next generation machine, which is based on the principal viewpoint of the use of a short wavelength, does not proceed so well.
It is conceived to increase the numerical aperture N.A., as another method to increase the resolution R. However, if N.A. is increased, there is a demerit that DOF of the projection optical system is decreased. DOF can be roughly classified into UDOF (User Depth of Focus: a part to be used by user: for example, difference in level of pattern and resist thickness) and the overall focus difference of the apparatus itself. Up to now, UDOF has contributed to DOF in a greater degree. Therefore, the development of the exposure apparatus has been mainly directed to the policy to design those having a large DOF. Those practically used as the technique for increasing DOF include, for example, modified illumination.
By the way, in order to produce a device, it is necessary to form, on a wafer, a pattern obtained by combining, for example, L/S (line and space), isolated L (line), isolated S (space), and CH (contact hole). However, the exposure parameters for performing optimum exposure differ for every shape of the pattern such as L/S and isolated line described above. For this reason, a technique called ED-TREE (except for CH concerning a different reticle) has been hitherto used to determine, as a specification of the exposure apparatus, common exposure parameters (for example, coherence factor .sigma., N.A., exposure control accuracy, and reticle drawing accuracy) so that the resolution line width is within a predetermined allowable error with respect to a target value, and a predetermined DOF is obtained. However, it is considered that the following technical trend will appear in future.
(1) In accordance with the improvement in process technology (improvement in flatness on the wafer), the difference in pattern level will be progressively lowered, and the resist thickness will be progressively decreased. There will be a possibility that the UDOF may change from an order of 1 .mu.m.fwdarw.0.4 .mu.m.
(2) The exposure wavelength changes to be short, i.e., g-ray (436 nm).fwdarw.i-ray (365 nm).fwdarw.KrF (248 nm). However, investigation will be made for only a light source based on ArF (193 nm) in future. Further technical hurdle is high. Thereafter, the progress will proceed to EB exposure.
(3) It is expected that the scanning exposure such as those based on the step-and-scan system will be predominantly used for the stepper, in place of the stationary exposure such as those based on the step-and-repeat system. The step-and-scan system makes it possible to perform exposure for a large field by using a projection optical system having a small diameter (especially in the scanning direction), in which it is easy to realize high N.A. corresponding thereto.
In the background of the technical trend as described above, the double exposure method is reevaluated as a method for improving the limiting resolution. Trial and investigation are made such that the double exposure method will be used for KrF exposure apparatus and ArF exposure apparatus in future to perform exposure up to those having 0.1 .mu.m L/S. In general, the double exposure method is roughly classified into the following three methods.
(1) L/S's and isolated lines having different exposure parameters are formed on different reticles, and exposure is performed for each of them on an identical wafer under an optimum exposure condition.
(2) For example, when the phase shift method is introduced, L/S has a higher resolution at an identical DOF as compared with the isolated line. By utilizing this fact, all patterns are formed with L/S's by using the first reticle, and L/S's are curtailed for the second reticle to form the isolated lines.
(3) In general, when the isolated line is used, a high resolution can be obtained with a small N.A. as compared with L/S (however, DOF is decreased). Accordingly, all patterns are formed with isolated lines, and the isolated lines, which are formed by using the first and second reticles respectively, are combined to form L/S's.
The double exposure method described above has two effects of improvement in resolution and improvement in DOF.
However, in the double exposure method, the exposure process must be performed several times by using a plurality of reticles. Therefore, inconveniences arise in that the exposure time (T4) is not less than two-fold as compared with the conventional apparatus, and the throughput is greatly deteriorated. For this reason, actually, the double exposure method has not been investigated so earnestly. The improvement in resolution and depth of focus (DOF) has been hitherto made by means of, for example, the use of an ultraviolet exposure wavelength, modified illumination, and phase shift reticle.
However, even when the contrivance is made, for example, for the exposure wavelength made to be ultraviolet, the modified illumination, and the phase shift reticle as described above, it is difficult to realize a resolution line width of 0.1 .mu.m which is the target for the next generation machine. Therefore, it is doubtless that the realization of exposure up to 0.1 .mu.m L/S based on the use of the aforementioned double exposure method for the KrF or ArF exposure apparatus makes a prevailing choice for the development of the next generation machine aimed at mass production of 256 M or 1 G DRAM.
It is conceived, as a means for further improving the resolving power based on the use of the double exposure method, to perform the double exposure by using the modified illumination as described above. In the case of the conventional modified illumination method, it is possible to improve the resolution and the depth of focus (DOF) for L/S and isolated L in a predetermined direction. However, the conventional modified illumination method is inconvenient in that the resolution and the depth of focus are considerably deteriorated for a pattern disposed in a direction perpendicular to the predetermined direction. Almost all such inconveniences can be dissolved by simultaneously performing modified illumination in two orthogonal axial directions. However, when each L pattern is inspected, the image is markedly deteriorated (for example, the edge portions become dull and tapered) at both end portions of the pattern (portions at which the two-dimensional edge exists). Therefore, an inconvenience arises in that the improvement in accuracy cannot be expected all the more as compared with a case in which exposure is performed by using a zonal type illumination method.
The double exposure method involves another inherent problem. That is, this method suffers from an inconvenience that the throughput is necessarily lowered, because it is necessary to perform the exposure process two times or more by using a plurality of reticles as described above. In this case, the inconvenience is not limited only to the increase in actual exposure time. In the conventional technique, since one sheet of reticle is placed on the reticle stage, the following inconveniences also arise when the double exposure method is carried out. That is, the throughput is lowered in an amount corresponding to the necessity to successively repeat a series of sequence in which (1) the reticles stored in a reticle library are taken out one by one by using a reticle loader or the like to perform reticle exchange with respect to the reticle stage, (2) the reticle is subjected to positional adjustment (alignment), (3) the exposure process is thereafter performed by using the reticle, and then the process routine returns to the step (1) again to exchange the reticle. Therefore, it is demanded to improve the throughput even to some extent by shortening the exchange time for the reticle when a plurality of reticles are exchanged and used.
It is conceived, as a method for shortening the reticle exchange time, to place a plurality of reticles on a reticle stage. However, if such a method is adopted, an inconvenience also arises in that the stage becomes to have a large size, and its positional control performance is deteriorated especially in the case of a scanning type exposure apparatus.
Further, it is necessary to make a special design concerning the management of the plurality of reticles, when the plurality of reticles are used as a set as in the double exposure method as described above.